union和union all

1、union和union all 将两个表直接纵向合并。

2、对于两个表中重复的行，union取其中一个，union all都会显示

3、union和union all直需要保证两个表的列数相同，列名可以不同。

 

 

 

 

with t as
(
select requester_id,accepter_id
from request_accepted
union all
select accepter_id,requester_id
from request_accepted
)

select requester_id id,count(*) num
from t
group by requester_id
order by num desc
limit 1;